Assessing Variability and Interactions of PM2.5-Bound Components in an Indian Desert–Urban Mixed Region

Abstract This study presents a comprehensive characterization of fine particulate matter (PM₂.₅) in an arid desert region of western India influenced by urban emissions. One-year long monitoring revealed that ~ 50% of days exceeded daily average NAAQS limit, underscoring persistent air quality challenges in this environment. Strong seasonality was observed, with organic carbon (OC) and elemental carbon (EC) peaking during post‑monsoon (15 ± 17 μg m⁻ 3 ) and winter (6 ± 4 μg m⁻ 3 ), and reaching their lowest levels in monsoon (0.5 ± 0.9 μg m⁻ 3 ). Water‑soluble inorganic ions dominated PM₂.₅ mass in colder seasons, contributing 51% (45.4 ± 31.8 μg m⁻ 3 ) in winter and 42% (32.3 ± 18.5 μg m⁻ 3 ) in post-monsoon, driven by aqueous‑phase chemistry under ammonia‑rich conditions. In contrast, neutralization during dust‑prone warmer months was regulated by crustal cations, with saline soils and playa lakes of the Thar Desert consistently supplying $$NaCl$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>NaCl</mml:mi> </mml:mrow> </mml:math> and $${Na}_{2}{SO}_{4}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>Na</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mrow> <mml:mi>SO</mml:mi> </mml:mrow> <mml:mn>4</mml:mn> </mml:msub> </mml:mrow> </mml:math> . Secondary reactions produced hygroscopic salts such as $${Ca(NO3)}_{2}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mrow> <mml:mi>C</mml:mi> <mml:mi>a</mml:mi> <mml:mo>(</mml:mo> <mml:mi>N</mml:mi> <mml:mi>O</mml:mi> <mml:mn>3</mml:mn> <mml:mo>)</mml:mo> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> </mml:math> , deliquescence at &gt; 10% RH, which could catalyse CCN and denser haze formations. During monsoon, favorable humidity and temperature promoted photochemical transformation of volatile organic fractions into water‑soluble organic carbon (WSOC, ~ 66% of OC), highlighting the role of OH radical chemistry in enhancing aerosol solubility. Overall, this work provides the first long‑term dataset of PM₂.₅ chemistry in a desert–urban mixed region of India. The findings reveal distinct seasonal pathways of aerosol formation and neutralization, demonstrate the climatic relevance of desert dust–urban interactions, and highlight the role of local sources in shaping PM₂.₅ chemistry. These insights offer critical inputs for air quality management and climate models, enabling more accurate region‑specific climate predictions. Graphic Abstract